The present invention relates to radio frequency mixers for electrical circuits such as transmitters and receivers. More particularly the invention relates to mixers having reduced LO leakage, mixers having means to reduce LO leakage, methods for producing such mixers and methods for reducing LO leakage of mixers.
Mixers are used in a variety of electrical circuits including transmitters and receivers. The demand for such devices has encouraged much activity and competition to produce devices of higher quality at lower prices. Also the increased demand has led in greater use of radio frequencies and the need to transmit and receive signals of greater purity, that is, signals which are contained completely within a particular bandwidth and without any accompanying impure signals which may interfere with bandwidths of other users. For example, a spurious emission is an emission by a user outside the section of the frequency spectrum licensed to that user. Since spurious emissions may affect the transmissions of other licensed users, they are not only undesirable, they may contravene transmission rules. It is well known that mixers routinely produce leakage which, if not dealt with properly, will result in spurious emissions.
Mixers are used in circuits to help process radio signals. A mixer is a radio frequency (RF) circuit that is used to change the frequency of an incoming signal to a different frequency, which may be preferred for signal processing or transmission. Mixers are used in many radio frequency circuits including transmitters and receivers and can ideally be considered as a controlled switch that performs a multiplication of a carrier or local oscillator (LO) signal with an incoming signal.
In practise, such a mixer is commonly implemented as a combination of a switching circuit and an input transconductance circuit. One example of a typical switching circuit and input transconductance circuit is a quad and a differential pair respectively. A diode ring circuit and a balun circuit are another example of a typical switching circuit and input transconductance circuit respectively. The quad switching circuit consists of four transistors connected so that the carrier or LO signal is applied to the bases, and the emitters are connected to the two output branches of the differential pair. The data input signal is applied to the bases of the differential pair with the resulting output current being applied to the emitters of the quad switching circuit. The quad switching circuit then changes the output polarity of the mixer, resulting in a frequency conversion of the output current at the collectors of the quad switching circuit. Ideally, each of the components in one branch has an identical counterpart in the other branch such that the electrical characteristics of one branch are identical to the electrical characteristics of the outer branch. Hence, a current corresponding to the input signals is generated in each branch of the differential pair, and is subsequently multiplied with the LO carrier signal in the quad switching circuit to produce a resulting output signal. In a circuit where the components in one branch of the differential pair are identical to the components in the other branch, the desired output signal is free from LO leakage spure. However, output signals free from LO leakage are difficult to generate because the semiconductor fabrication process inherently introduces variations in the values of the mixer circuit components, leaving the branches of the differential pair mismatched.
It is well known in the art that mixers have an unwanted leakage component, mostly due to process variations during the manufacturing process as mentioned above. Thus, although they provide the desired and expected mixer output signals, the output signal that leaves the mixer almost invariably includes a signal leakage component, specifically, an unwanted component at the LO frequency, called the local oscillator (LO) leakage of the mixer. LO leakage occurs when a DC current differential, also known as a current imbalance or offset current, is formed between the mismatched branches of the differential pair during mixer operation.
Systems such as transmitters and receivers for example, include mixers which must be carefully designed to reduce or remove the LO leakage so that it does not render the circuit impractical, by infringing on other bandwidths for example. In the past, various attempts have been made to eliminate or reduce mixer leakage. One of the most common methods is to use filters following the mixer to filter out the LO leakage. However, the LO leakage is usually produced at a frequency close in the desired target frequency and is consequently difficult to filter out. Additional signal processing employing corrective feedback loops also poses problems. Such loops add a risk of instability to the system, increase the circuit complexity and may require additional software coding to control the circuitry.
However, even the solution of using filters poses serious cost and design problems. For example, although the filter used may reduce the LO leakage signal, it also reduces the strength of the desired signal due to the insertion loss of the filter. The filtered signal then requires further amplification to increase its strength. However, further amplification also amplifies any remnant component of the leakage signal, and additional filtering may be required. Every additional component used in the circuit naturally increases the cost and complexity of the circuit design, which adds further problems for the designer to deal with in order to construct an efficient, economical and practical circuit. Indirect costs, such as higher power consumption and circuit board real-estate consumption by the additional components are also incurred.
During chip manufacturing, although the same process may be used to fabricate each mixer, the output LO leakage will vary from one mixer to another, even within the same batch due to statistical variation of component characteristics. It is standard practice to test each device against desired specifications after manufacturing, and if it does not meet the specifications, it must be discarded. If, after manufacturing, one was to measure the power of the LO leakage, it would be found that the power measurement would vary in a statistical manner. Once a specification for LO leakage has been set, it would be found that the power of the unwanted LO leakage signal from the LO port to the output port would be insignificant within a certain range of DC current differential. Depending on the intended use of the mixer, this range of acceptability may be large or small. However, usually the acceptability would represent a range and thus as long as the LO leakage power could be sufficiently reduced, the mixer would be found acceptable. For many intended uses, when a batch of mixers is manufactured, there will usually be at least several mixers which have acceptably low LO leakage so that no modification is needed. However, for the remainder, if no modification is possible, they are discarded. Thus it usually happens that for a given commercial product with a particular circuit, there is a certain range of acceptability for mixer LO leakage. If a significant proportion of the manufactured mixer device does not meet the required specifications of the system in which it is intended for and are discarded, the overall cost of the final product/commercial article increases.
It is therefore desirable to provide a simple, low-cost circuit and method for reducing mixer LO leakage in radio frequency circuits.
It is an object of the present invention to obviate or mitigate at least one disadvantage of previous systems and methods for dealing with mixer leakage.
In a first aspect, the present invention provides a mixer for a radio frequency circuit. The mixer has an input circuit, a switching circuit and a compensator circuit. The input circuit is connected to complementary data inputs for providing currents on a first and second output node respectively. The switching circuit is connected to complementary local oscillator inputs and the first and second output nodes for driving complementary mixer output nodes. The compensator circuit controls a DC current differential between the first and second output nodes to reduce local oscillator leakage power at the complementary mixer output nodes.
In an embodiment of the present invention, the input circuit includes a differential pair having first and second current branches, and the switching circuit includes a switching quad. In an aspect of the present embodiment, the compensator circuit is connected to the first and second output nodes, and the compensator circuit injects current into at least one of the first and second output nodes. In the present aspect, the compensator circuit includes at least one transistor and at least one current sink for controlling the injected current from the current sink, where the at least one transistor receives a bias voltage. The at least one transistor can be an npn BJT, and the voltage supply can be VEE. Alternatively, the at least one transistor can be a pnp BJT, and the voltage supply can be VCC. In other aspects of the present embodiment, the input circuit includes a balun circuit or a common base pair. The common base pair includes first and second current sinks which are variable for controlling the DC current differential.
In another aspect of the present embodiment, the first and second branches receive current from first and second current sinks respectively, where the first and second current sinks are variable for controlling the DC current differential.
In another aspect of the present embodiment, the compensator circuit is connected to the complementary data inputs, and changes a DC voltage level of the input signals. In the present aspect, the compensator circuit includes at least one resistor for selectively connecting to ground and an anti-fuse for selectively connecting the at least one resistor to ground. The compensator circuit can alternatively include a buffer circuit configured as a differential pair.
In yet another aspect of the present embodiment, the compensator circuit selectively changes the resistance of at least one of the first and second current branches.
In another embodiment of the present invention, the compensator circuit includes at least one power dissipation element for generating heat. The at least one power dissipation element includes at least one resistor connected to VCC for selectively connecting to ground, and an anti-fuse for selectively connecting the at least one resistor to ground. In another aspect of the present embodiment, the at least one power dissipation element includes at least one transistor and at least one current sink serially connected between the resistor component and ground, where the at least one transistor receives a bias voltage.
A further aspect of the present invention provides a radio frequency transmitter. The radio frequency transmitter includes a digital to analog converter, a filter, a mixer and an antenna. The digital to analog converter receives digital data and generates an analog data signal corresponding to the digital data. The filter receives the analog data signal and provides a filtered data signal to the mixer. The mixer changes the frequency of the filtered data signal in accordance with a high frequency local oscillator, and provides a high frequency data signal. The antenna transmits the high frequency data signal. The mixer has an amplifier circuit, a switching circuit and a compensator circuit. The amplifier is connected to complementary data inputs circuit for providing currents on a first and second output node respectively. The switching circuit is connected to complementary local oscillator inputs and the first and second output nodes for driving complementary mixer output nodes. The compensator circuit is operatively connected to the amplifier circuit for reducing a DC current differential between the first and second output nodes to reduce local oscillator leakage power at the complementary mixer output nodes.
In yet a further aspect of the present invention, there is provided a method for mixing a data input signal with a local oscillator frequency for converting the frequency of the data input signal to the frequency of the local oscillator. The method includes the steps of providing a local oscillator signal to a mixer, and applying a compensator current to the mixer to reduce local oscillator leakage.